The present invention relates to an integrated multisensor (a composite sensor) which is used in a pressure and differential pressure transmitter for detecting a flow (or the quantity of flow) or a pressure in a chemical plant or the like and in which the characteristic of a static pressure sensor upon differential pressure load is improved, and also relates to an intelligent differential pressure transmitter and a plant system which use such an integrated multisensor.
In the conventional sensor used in a sensing part of an intelligent differential pressure transmitter, differential pressure, static pressure and temperature sensors are provided on one substrate and the three kinds of sensors make compensation for each other to measure a differential pressure with a high precision. For example, U.S. Pat. No. 4,530,244 has proposed a structure in which a sensor for detecting the influences of temperature and static pressure is provided on a thick thickness portion and a sensor for detecting a differential pressure is provided on a diaphragm. Also, JP-B-62-22272 has disclosed the combination of a sensor for removing the influence of a static pressure and a differential pressure sensor but the influence of a differential pressure acting on a static pressure sensor is not discussed. Namely, it is known that in the conventional integrated multisensor, the respective outputs of sensors cannot provide independent variations and have influence on each other.
The conventional differential pressure transmitter determines a pressure difference or differential pressure .DELTA.P produced when a liquid pressure-transmitted by a pressure pump passes through an orifice provided in a pipeline of a chemical plant or the like, thereby detecting the quantity of flow Q according to the following equation: ##EQU1## where k is a constant determined by Reynold's number and diameter of the orifice. As mentioned above, since the output of a differential pressure sensor is influenced by a static pressure P.sub.s and a temperature T. Therefore, in the measurement of the quantity of flow, the static pressure P.sub.s exerted on the pipeline and a change of the ambient temperature turn out disturbance to the integrated multisensor and hence the influences thereof should be removed to the utmost.
In a differential pressure transmitter using the conventional integrated multisensor, a static pressure, a temperature and a differential pressure are detected by indivisual sensors. As mentioned above, since each sensor detects a corresponding variation which includes the other variations in an added form. Therefore, a complicated procedure for correction is carried out to remove the influences of the other variations.
Also, in the case where a differential pressure detecting diaphragm and a static pressure detecting diaphragm are provided in one semiconductor substrate, there is a problem that an interval between both the diaphragms becomes narrow (during an etching process), thereby decreasing the adhesive length to a fixing base which supports the semiconductor substrate.
Further, in the case where a differential pressure detecting diaphragm with a central rigid plate is formed by use of anisotropic etching, there is a problem that a quadrangular central rigid body surrounded by &lt;111&gt; faces is most easily formed with the result that a stress is concentrated at each corner portion, thereby decreasing the pressure durability (tight ability).
Furthermore, if a differential pressure detecting diaphragm and a static pressure detecting diaphragm have the same plate thickness, there is a problem that when the size of a semiconductor substrate is to be made small, the static pressure detecting diaphragm becomes too small thereby making it difficult to arrange a gage resistor(s) on the static pressure detecting diaphragm.